The present invention relates to device for dispensing, injecting, administering, infusing or delivering substances, and to methods of making and using such devices. More particularly, it relates to a device for administering an injectable product which enables a dose to be freely selected by a user of the device. The injection device is suitable for situations in which the user self-administers the product and is able to select, i.e. set or choose, a dose individually with every administration. More particularly, the injection device is of the type suitable for administering insulin as a diabetic treatment, or for administering growth hormone.
Patent specification EP 0 713 403 A1 discloses a syringe for administering liquid pharmaceutical mixtures and other liquids which allows a dose of liquid to be administered per injection to be set once for every injection. Specifically, a setting is made by a pharmacist. However, in the case of a patient who then has to use the syringe to self-administer the pharmaceutical liquid, it is difficult to change the dose once it has been set. The intention is to prevent an incorrect dose from being administered with the syringe. A syringe of this type is not effectively useful in some treatments because an optimum or requisite dose may vary depending on, for example, the time of day, sporting activities or the consumption of meals.
Injection devices which satisfy requirements of variable doses are known from patent specifications WO 97/36625 and DE 199 00 792 C2, for example. These two specifications relate to injection devices, each of which has a conveying mechanism for dispensing the product, and a dose metering mechanism for setting the product dose which can be conveyed and dispensed by the conveying device during a subsequent injection. The conveying mechanism comprises a plunger, the forward stroke of which conveys the product from a product reservoir, a plunger rod and a drive member for the plunger rod. The drive member and the plunger rod engage with one another so that a forward movement of the drive member causes the plunger rod to move in the same way but the drive member performs a reverse movement in the opposite direction until it reaches a trigger position, from which another injection can be initiated. The trigger position is determined by means of the dose metering mechanism, which forms an adjustable dose metering stop for the drive member. Although the known devices have proved to be efficient in practice, they could still be improved to make them more reliable in terms of minimizing the risk of incorrect doses.